The aims of developments in internal combustion engines are to lower the fuel consumption and to reduce exhaust-gas and noise emissions. In order to achieve both high torque values and power values and low part-load consumption in spark-ignition petrol engines, in the case of four-stroke reciprocating-piston internal combustion engines with direct fuel injection into the combustion space a mixed operating mode within the engine characteristic diagram is implemented in that, in the upper rotational-speed range and load range, a homogeneous operating mode with a stoichiometric air ratio in combustion is carried out and, in the middle and lower rotational-speed range and load range, a stratified-charge operating mode with a largely unthrottled air supply and with a stratified charge is carried out.
In order to achieve a homogeneous fuel/air mixture, fuel is injected directly into the combustion space of the reciprocating-piston internal combustion engine at a very early stage, specifically even during the suction stroke of the reciprocating piston. As a result, the intake air quantity is efficiently utilized and a high average pressure and consequently a high torque value are achieved. Under part-load, quality control becomes possible by charge stratification, in that the fuel is injected only very late into the combustion space during the compression stroke of the reciprocating piston. In practice, reciprocating-piston internal combustion engines for motor vehicles are operated essentially in the part-load range. The characteristic-diagram range for the essential driving situations should therefore coincide throughout the characteristic-diagram range with a charge stratification which is beneficial in terms of consumption.
In combustion methods for reciprocating-piston internal combustion engines with a direct injection of petrol fuels, a distinction is made between three basic patterns, to be precise a jet-controlled method, a wall-controlled method and an air-controlled method. Depending on the method, the properties of the fuel jet, the combustion-space geometry and the charge movement are important influencing variables. A jet-controlled method is characterized by the close spatial relationship between the fuel injection device and the spark plugs. The fuel cloud injected into the combustion space via the fuel injection device forms a compact zone, into which swirled air is mixed to a limited extent. For controlling the stratification profile, the spatial arrangement of the fuel injection device and spark plug must be coordinated exactly, since an ignitable mixture must be present at a spark plug at the ignition point.
German Published Patent Application No. 197 30 908 describes a four-stroke reciprocating-piston internal combustion engine. It is operated in a stratified-charge operating mode in part of its operating characteristic diagram, preferably during idling and in the low rotational-speed and load range, and is operated in a homogeneous operating mode in a stratified-charge operating mode and in the remaining characteristic diagram. In the stratified-charge operating mode, the necessary fuel quantity is injected into the combustion space at the end of the compression stroke, with the result that a mixture cloud with strata having different fuel/air ratios. In this case, an ignitable mixture occurs at the edge of the injection cone, in cooperation with a combustion-space flow, and is ignited by an ignition spark between the electrodes of a spark plug. If the fuel/air mixture is too rich or too lean, that is to say does not correspond essentially to a stoichiometric mixture, it is not ignitable. So that the ignition spark reliably ignites the cylinder charge, the spark duration is prolonged in the stratified-charge operating mode, as compared with the homogeneous operating mode.
When the internal combustion engine is operated in the full-load mode with homogeneous mixture formation, the fuel is injected into the cylinder predominantly as early as the intake stroke, only ignitable mixture being present between the electrodes at the ignition point due to the good mixture preparation, and a minimal spark duration of about 0.1 milliseconds being sufficient for ignition. A control unit is provided, which, on the basis of at least one operating parameter of the internal combustion engine, generates a control signal with evidence of the spark duration and supplies it to the ignition system.
For a misfire-free stratified-charge operation of the internal combustion engine, it is necessary for the spark plug to project into the edge region of the injection cone, since ignitable mixture occurs in this region only. If, for example because of carbonization of the injection nozzle, there is an unintended change in the cone angle at which the injection cone of the fuel extends into the combustion space, combustion misfires occur, since the ignition spark is no longer located in the region of an ignitable mixture.
It is an object of the present invention to provide a four-stroke reciprocating-piston internal combustion engine with spark ignition, which is operated in a homogeneous mode and in a stratified-charge mode, in which the ignition reliability of the fuel/air mixture in the combustion space may be improved.